1. Field of the Invention
The invention pertains to the art of image processing and a digital signal processing method and apparatus therefor and, more particularly, to an apparatus for scaling, i.e., enlarging or reducing, an image.
2. Description of Related Art
In digital image processing, the ability to electronically magnify or reduce an image is almost uniformly required as a feature of conventional imaging devices such as copiers and printers. Selected scaling of the image can be limited to a few predetermined choices, but present market tendencies are to demand virtually unlimited scaling options, particularly in the higher quality copying machines. The hardware circuits that have been designed to implement such scaling have been required to capture and process the pertinent image signals with reduced processing time and with minimal storage requirements, for improved operating efficiency and minimization of hardware costs.
The present digital scaling system is directed to a method and control circuit for implementation thereof for selected enlargement, reduction and anamorphic scaling of an image. For the subject disclosure, "anamorphic" is intended to mean the ability to scale so that the images are enlarged in one dimension and reduced in another dimension; and can also include enlargement or reduction in one dimension with no scaling in the other dimension.
The description includes references to slow-scan and fast-scan digital image data when discussing the orientation of selected window coordinates and sampled image data signals used by control circuitry. For purposes of clarification, data collected along a fast scan direction is intended to refer to individual pixels located in succession along a raster of image information. On the other hand, data collected in the slow-scan direction refers to data derived from a common raster position across multiple rasters of image information. As an example, fast-scan data would refer to the sequential signals collected along the length of the linear photosensitive array during a single exposure period, and is commonly referred to as a raster of data. Slow-scan data would be used to describe signals captured from a plurality of elements along a linear photosensitive array, as the array was moved relative to a document. Analogously, your eyes move in the fast-scan direction relative to this page as you read along each line; when your eyes move to the next line down, they have moved in the slow-scan direction.
Various scaling techniques are known for manipulating static images which are later output onto such media as the screen of a personal computer or onto printed paper. Practically all techniques calculate the value of a new pixel based on the values of neighboring original pixels. For example, linear interpolation uses a matrix of adjacent pixels where the nearest neighbors in the slow scan and fast scan direction define the matrix employed in the calculation of the new pixel value.
A known problem with regard to prior interpolation scaling systems is, particularly with regard to reduction, that such systems operate in a manner to cause an undesired loss of data. Simply stated, linear interpolation throws away data by discarding the values of intermediate original pixels during the calculation of values for the pixels of the reduced image. The loss of such data is a problem sought to be overcome by higher quality imaging systems.
The present invention contemplates a new and more efficient scaling process, which overcomes the above-referenced problem and others to combine a new pixel window averaging process for reduction scaling, in conjunction with linear interpolation for enlargement scaling, to obtain a higher image quality processing system for a digital scaling system.